Astronomical
dictionary

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This is by no means a comprehensive
glossary of astronomical terms however it should provide some assistance to
novices in particular. Click on the
symbol for a diagram or picture associated with that word. Cross references
within the glossary are indicated by hyperlink
text. Click on the letters below to go to the first entry of alphabetical sections.

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Click the link to go to the contents
page (lists all of the words in the dictionary) or click on the letters
below to go to the first entry of each alphabetical section. Click the link
to get back to the site entry page.

Netscape users: Please
note that some of the text formatting does not seem to render correctly. I do
not know why, it works fine in IE.

Click here
to visit Aries
on line, the Derby and District Astronomical Society magazine. It
contains a variety of articles including beginners materials.

Aberration (of starlight)

The
apparent displacement of a star from it's true position in the sky.
It is caused by a combination of the motion of the Earth in orbit round
the Sun (about 30 km per sec) and the finite velocity of light (299,792.5
km per sec or , if you prefer imperial units, 186,252.5 miles per second).
During the year a star will seem to move in a small ellipse
around its true position. This is called annual aberration. There is
also a very much smaller daily effect caused by the rotation of the
Earth. This is called diurnal aberration. The maximum displacement is
20.5 seconds of arc. This number is called the constant of aberration.
For a much more thorough treatment click here.

Absolute Magnitude

The
magnitude
a star would seem to have if viewed from a standard distance of 10 parsecs
(32.6 light years).

Accretion Disc

A disc of
material from which larger bodies may be formed under the influence of
gravity. Also the rotating disc of material formed around a Black
Hole.

The faint luminosity
of the night sky, due to the Earth's upper atmosphere.

Airy Disc

The apparent
size of a star's disc produced by a perfect optical system.

Albedo

A measure of how
reflective a body is. Albedo is expressed as a percentage, the higher
the percentage, the higher the albedo and therefore the more reflective
the object is.

Altazimuth Mount

A type of telescope
mount, which allows the instrument to be moved freely in declination (altitude)
and right ascension (azimuth).

Altitude

The angle of
elevation (height in degrees) of a star or other astronomical object above
the observer's horizon.

Analemma

If you take a
picture of the Sun at the same time each week, from exactly the same position,
on a single frame of film, the figure of eight shape that results is called
the analemma. See www.analemma.de
for more detail and photos.

Angular Distance

The apparent
distance between two celestial objects. It is measured in degrees, arcminutes
and arcseconds. On average,
the distance from your thumb tip to the tip of your little finger of your
outstretched hand at arms length is 20 degrees. The width of your palm
will be about 12 degrees and the width of the tip of your little finger
is about 1 degree. The angular diameter of the Moon (and the Sun) is more
or less 1/2 degree.

Aperture

The diameter of the main
lens or mirror of a telescope.

Aphelion

When something
is orbiting the Sun, this is the point of the orbit that is furthest from
the Sun. See also perihelion.

Apogee

When something
is orbiting the Earth, this is the point of the orbit that is furthest
from the Earth. See also perigee.

Apennines

One of the most
noticeable mountain range on the Moon, next to Mare Imbrium (Sea of Showers).

Aperture Synthesis

A technique
used in radio astronomy where by an array of radio telescope dishes are
used together, effectively giving the observer a much larger dish size
e.g. the VLA in New Mexico (visit the Aries
on line site to read an article about the VLA).

Appulse

The apparent close
approach of one celestial body to another. This is a line of sight effect.

A small measure
of angular distance.
There are 60 arcseconds in one arcminute and therefore 3600 arcseconds
in one degree.

Areography

The physical
study of Mars.

Ashen Light

When the plant
Venus appears as a crescent, the night side sometimes appears dimly luminous.
This is the Ashen light. The reason for the Ashen light is not understood.

Asterism

A distinctive
group of stars that is not one of the recognised constellations.
The Plough is a good example, it is not a constellation, it is an asterism
that is part of the constellation Ursa Major.

Asteroids

These are relatively small bodies that orbit
the Sun mainly between the orbits of Mars and Jupiter. They are though
to be material of the Solar System that never accreted to form a planet.
There are others who believe that they are the remains of a somehow
destroyed planet that lay between Mars and Jupiter. In recent years
it has been realised that there are a number of asteroids that cross
the orbit of the Earth. Some of these are quite large and would cause
great problems if they impacted the Earth. Life on this planet could
conceivable be eradicated by a large enough impact. It is now generally
accepted that mass extinctions of life in the past e.g. the demise of
the dinosaurs, could have been caused by such an impact. There is now
an observing programme to locate and evaluate the dangers of this hazard
(NEAR). I have put some links to books on the subject below.

Astrology

A pseudo science
based on ignorance and superstition in which the stars are believed to
influence daily personal events on the Earth!

Astrometric Binary

A binary
system in which the fainter component can not be seen, but is observable
from the gravitational effects on the proper
motion of the brighter companion.

Astrometry

The branch
of astronomy dealing with the movements and positions of celestial bodies.

Astronomical Twilight

The period between
sunset and the time when the sun has dropped 18 degrees below the horizon.

Astronomical Unit

The mean (average)
distance of the Earth to the Sun is termed 1 Astronomical Unit (1 AU).
It is a convenient way of describing distances within our Solar System.

Astrophysics

The branch
of astronomy that deals with the physics and chemistry of the stars.

Atmosphere

The gaseous
mantle surrounding a planet or other body. It is thought that the atmosphere
of the Earth is a secondary atmosphere. The theory is that the original
(primary) atmosphere was lost during the T-Tauri
stage of the Sun's evolution. Volcanoes gradually replaced this with an
atmosphere of methane, carbon dioxide and water vapour. The current atmosphere
evolved from this. The oceans were formed as the water vapour condensed
as Earth cooled down. When plants containing chlorophyll evolved, they
used the carbon dioxide for photosynthesis and introduced oxygen into
our atmosphere.

Atmospheric refraction

This
is the bending of light the Earth's atmosphere. This causes an increase
in the apparent height (altitude)
of an object above the horizon. This increase is zero at the zenith (because
the observer is looking out from the Earth at 90 degrees) to about half
a degree (the width of the full Moon) at the horizon.

Atom

The smallest unit
of a chemical element which retains its own characteristics. An excellent
book on the atoms and Einstein's theory of relativity is "Mr
Tompkins in paperback" by George Gamow

Aurora

A glow in the northern (aurora borealis) or the
southern (aurora australis) sky caused by particles from the solar
wind interacting with the atmosphere.
The particles are held in the Van
Allen belts. When there is a solar flare, there is a huge increase
in the density of the Solar wind. This overloads the Van Allen belts
and causes high energy particles to spill into the atmosphere. As these
interact with the gases of the atmosphere, light of different wavelengths
is emitted and we see this as an aurora.

Book: The nature of the Aurora was worked out by the Norwegian Scientist
Kristian Berkeland. I was riveted by the book Northern
Lights, an account of how the mysteries of the Aurora were unravelled
and learn about the man who invented a weapon developed in the USA's
SDI (Star Wars) early in the twentieth century.

Azimuth

The angle measured
from the south point of the horizon toward the west to a point at the
foot of a star's vertical circle. See also celestial
sphere.

Baily's Beads

Brilliant points of light
along the edge of the moon disc, just before and just after a total
solar eclipse.

Put a Barlow lens
between the main mirror or lens of your telescope and its eyepiece and
increase the magnification. Effectively it increases the focal length
of the main mirror or lens. The most usual magnification for a Barlow
lens is X2. In practice, they are rarely used since they cause a large
light loss in the telescope. It is a cheap way of getting a short focal
length eyepiece. Better to spend more on an eyepiece, you get better quality.

Big Bang Theory

The
theory that the universe came into existence at one set moment in time,
between 15,000 and 20,000 million years ago.

Named after
the first discovered member of this type of object. It appeared to be
a variable star at first but it was realised that they were much more
luminous than stars. They are less luminous than quasars and one idea
is that a BL Lacertae object is a quasar seen from a narrow angle.

Black Body

A body that
absorbs all the radiation that it receives, giving it an albedo
of zero i.e. it is 100% efficient at absorbing radiation. A black body
also radiates with 100% efficiency, it is therefore a theoretical object
although many stars come very close to this theoretical position.

Black Drop

An appearance
seen at the beginning of a transit
of Venus. As the planet
moves across the Sun's disc it seems to draw a strip of blackness after
it.

Black Dwarf

A dead star,
which has used up all its reserves of energy. The ultimate fate of a White
Dwarf.

Black Hole

A localised
region of space from which not even light can escape, due to a super massive
star collapsing in on itself. This is because the gravitational field
of a black hole is so strong that the escape
velocity is greater than the speed of light. Astronomers believe that
the location of large black holes can be observed because of radiation
emitted from the accretion
disc as matter is pulled into the black hole. The event
horizon marks the outer limits of these objects.

Blue shift

If an astronomical
body is moving towards the observer, the light will seem to be shifted
to the blue end of the spectrum.
The faster the movement, the greater the blue shift. It occurs because
the wavelength
of light is slightly compressed by the Doppler
effect as the body moves towards the observer. Blue shift is measured
by looking at the key spectral
lines. For an object moving towards the Solar System, they will appear
closer to the blue end than normal. The faster the object is approaching,
the greater the blue shift will be.

Bok Globules

The Dutch astronomer
Bart Bok first drew attention to these small black objects. They appear
in gaseous emission nebulae
and are thought to be protostars
that are still forming but have not yet become hot enough to shine.

Red
stars of spectral
types R and N, containing an unusual amount of carbon in their atmosphere.

Cassegrain Reflector

A type of reflecting
telescope where the main mirror has a central hole. Light from an object
is reflected off the primary mirror up to the secondary mirror and back
through the hole in the primary to be focused in the eyepiece tube. The
primary mirror is a spherical mirror and is therefore easier and cheaper
to make than a parabolic (slightly elliptical) mirror. Correction for
spherical aberration is made by having a parabolic secondary mirror. The
path of the light is also folded on itself which makes its tube much shorter,
lighter and more portable than a refracting or a Newtonian reflector of
the equivalent aperture.

Cassini Division

The principal
division in Saturn's ring system, separating ring A from ring B.

Catadioptic Telescope

A
type of telescope, which uses both refraction and reflection to form an
image at the prime focus. This type of telescope is generally based on
the Cassegrain design but
uses a corrector plate to prevent spherical aberration. This means that
both the primary and secondary mirror can be spherical rather than parabolic.
See Cassegrain Telescope
for the advantages.

Celestial equator

A great
circle on the celestial sphere that is midway between the two poles of
rotation. It really amounts to the projection of the Earth's equator onto
the sky. It can be thought of as an imaginary ring, 90 degrees from either
pole in the sky.

The angle that
is measured eastward along the ecliptic, from the vernal
equinox, to the foot of a circle that is perpendicular to the
ecliptic and passing
through the object.

Celestial sphere

This does not
actually exist. Early humans believed that stars were fixed to a crystal
sphere in the sky, at a great distance from Earth. This is because there
is no sense of distance in the night sky and the stars always seemed to
be fixed in position relative to each other. The idea is a handy one when
dealing with positioning and angles between objects in the sky. See also
celestial
latitude, longitude,
azimuth.

Cepheid Variable

An important type
of variable star. Stars of this type have short periods, from a few days
to a few weeks; these stars are perfectly regular. The period of a Cepheid
depends on its absolute magnitude. So, when the period of variability
is known, its absolute magnitude can be deduced. Comparing this to its
apparent magnitude (how bright it is when seen from Earth) gives the distance.
They are sometimes referred to as "standard candles" because
of their usefulness in determining the distances of galaxies. They are
named after the star Delta Cephei, which was the first one of this type
of star to be discovered.

Charge Coupled Device (CCD)

A very sensitive electronic
device used in astrophotography. For further details see the article in
Aries
on line.

Chromatic aberration

A
defect of a lens that creates a fringe of colour round an object.

Chromosphere

The
part of the sun's atmosphere lying above the bright photosphere, but below
the corona.

Circumpolar

This
is a region of the sky that is always visible round the celestial pole
closest to the observer. An object in this area will therefore never set,
at any time of the night (or day of course) and can be observed at any
time of the year e.g. the Plough asterism
is in the circumpolar region from the UK and can be seen in all four seasons,
Orion is not circumpolar and so can only be observed for part of the year.

Cluster (stars)

A
cluster is a group of stars whose members are genuinely associated. A
cluster of stars is formed from the same gas/dust cloud. There are two
main types: open
and globular.

Cluster (Spacecraft)

Four spacecraft that have
been placed in Earth orbit in order to study the interactions between
particles ejected
from the Sun and the Earth's magnetic field. The first attempt to put
Cluster into space in 1996 failed when the Ariane rocket exploded.

Collapsar

The
end product of a very massive star, which has collapsed to form a very
high density object.

Colour index

A
measure of a star's colour, which helps astronomers to tell its surface
temperature. It is the difference between the magnitude
of a star measured in two different areas of the spectrum. The areas are
B (blue), V (violet) and U (ultraviolet) regions. The B-V is the most
common index used and is close to zero for a white star. It is extremely
useful in the classification of stars, it can tell astronomers if the
star is a main sequence star, a giant star or a supergiant star.

Coma

There
are several astronomical meanings. It can be used to describe the hazy
looking patch that surrounds the nucleus of a comet or the blurred effect
surrounding the images of stars on a photographic plate, or in the observers
field of view in a telescope (or binoculars) due to defects in the lenses.

Comes

This
is pronounced 'Koh-meez' and means the fainter companion of a double
or binary star.

Comet

A body of our Solar
System. Comets are composed of rocks, dust and ices. They are believed
to come from the Oort cloud and possibly contain material from the earliest
days of the Solar System. As they approach the Sun, the ices begin to
evaporate forming a coma and one or more tails.

Conjunction

This
is when a planet has the same longitude as the Sun. A planet can only
be observed at conjunction if there is a total eclipse
or if it transits
the Sun. Unlike the other planets, Mercury and Venus have two types of
conjunction, inferior
when the planet lies between the Sun and the Earth, superior when it lies
on the opposite side of the Sun to the Earth.

Constellation

A pattern
of stars named after an object, animal or mythical person. The stars in
a constellation have no physical connection, they are a line of sight
effect.

Convection

The transfer
of energy by moving currents in gases or liquid. Partly responsible for
the transfer of heat from the interior of the Sun to the photosphere.

Copernican system

The system
proposed (in 1543) by Nicholas Copernicus in which the Sun is the central
body, with the Earth and the other bodies moving around it. This model
superseded the Ptolemaic system which had persisted for nearly 2000 years.
Copernicus' idea was not new, it had been proposed in about 300 BC by
Aristarchus of Samos, a Greek philosopher. Both astronomers have Lunar
craters named after them.

Corona

Faint extensions
of the Suns outer atmosphere seen during total eclipses.

Coronal hole

A region of
very low density and temperature in the Sun's corona. Electrified particles
can escape through coronal holes to produce the solar wind.

Cosmic rays

High speed
particles that reach the Earth from outer space.
Some low energy cosmic rays come from the Sun, but high energy cosmic
rays are thought to originate from outside of the Solar System. These
possibly originate from supernovae.
The highest energy cosmic rays may originate in quasars.

The maximum
altitude of a celestial body above the horizon. In other words, when an
astronomical body transits the meridian.

Dark Adapted

This is what
every astronomer's eyes should become before they begin observing. On
leaving a brightly lit area and entering a dark area, you will notice
a rapid increase in visibility over a short period of time. It takes about
20 - 30 minutes to become fully dark adapted. In low light levels, the
chemicals in the eye increase the ability to see faint objects. See also
the Purkinje
effect

Declination

The
angle north or south of the celestial equator to a star.

Doppler effect

The classic example is the change in tone
of the noise of a vehicle engine as it approaches, passes and leaves
the observer. As the vehicle approaches, more sound waves per second
enter the ears of the observer. The sound appears to be higher pitched
that if the vehicle and observer were stationary. This is because more
sound waves per second = a higher frequency and higher frequency sound
is heard as a higher pitch.

As the vehicle passes the observer and moves away, the engine tone
is heard to drop. This is because slightly fewer waves per second enter
the ear. Thus, the tone is heard to fall (lower frequency). The same
happens to all waves, including light waves and radio waves. In the
case of light waves, higher frequencies are bluer and lower frequencies
are redder. An object with a negative radial
velocity (moving towards the Solar System) will be blue
shifted and vice versa will be red
shifted.

Double star

When you observe
some stars through a telescope or binoculars, they appear to be two as
opposed to one when seen with the naked eye. Sometimes this is a line
of sight effect where the two stars are in reality totally unconnected
with each other. In other cases the two stars are a genuine pair, orbiting
one another (see binary
star).

Eccentricity

The
degree of flattening of an ellipse.
The term is applied to planetary orbits since these are ellipses.

Eclipse

The obscuring of one
celestial body by another either by passing directly in front of it
or by casting a shadow. Solar eclipses
can be total (whole Sun is obscured), partial (part of the Sun is obscured)
or annular (a complete ring of sunlight is seen). Lunar
eclipses are either total or partial.

This is the plane
of the Earth's orbit projected onto the celestial sphere. Effectively
this is the apparent path of the Sun through the sky. Since the main planets
of the solar system orbit in more or less the same plane, give or take
a few degrees, they are always found close to the ecliptic.

Ellipse

An oval. The planets
do not orbit in perfect circles, as was believed before Kepler's
Laws of planetary motion, but rather in slightly oval (elliptical) orbits.
Their eccentricity is
low so the orbits are not far off being circles. An ellipse has two foci
(plural of focus), in the Solar System, the Sun lies at one of these.

Elongation

These terms
are generally applied to Mercury and Venus. When seen from Earth, when
either planet reaches its greatest angular distance from the Sun, they
are said to have reached eastern (or western) elongation.

Escape velocity

The
velocity required by an object to escape the gravitational pull. The gravitational
attraction of a black hole is so great that an object would need to be
travelling faster than the speed of light in order to escape. That is
why black holes are essentially invisible, even light cannot travel fast
enough to escape the immense pull of the gravity of a black
hole.

Equinox

Literally "equal
night". In other words, in Spring (Vernal) or Autumn (Autumnal) when
the daylight hours are equal to the darkness hours. Technically speaking,
the point at which the ecliptic
and celestial
equator intersect.

Event horizon

The boundary
that marks the point of no return of a black
hole. Once light (or matter) passes this, the gravitational forces
are so strong that the escape velocity woild be greater than the speed
of light. See also Schwarzchild
radius.

Eyepiece

The combination
of lenses that magnifies the image formed by the objective (main mirror
or lens). There are different types of eyepiece, each has its merits.
The most common type seems to be the Plössl, which is a good general
eyepiece. Huygens and Kellner are the names of two other types of eyepiece.